The invention relates to motor vehicles in general, and more particularly to improvements in power trains for use in passeger cars, vans, trucks and/or other types of motor vehicles. Still more particularly, the invention relates to improvements in power trains of the type wherein an automated engageable and disengageable torque transmission system (such as a friction clutch, a lockup clutch, a magnetic powder clutch or the like) cooperates with a prime mover (such as an internal combustion engine) and/or with a variable-speed transmission (e.g., a manually shiftable or an automated transmission) to transmit torque, e.g., from a rotary output element (such as a camshaft or a crankshaft) of the prime mover to a rotary input element (e.g., a shaft) of the transmission.
A power train which embodies an automated torque transmission system (hereinafter called clutch for short) further comprises means for operating the automated clutch, and such operating means can comprise an electronic signal receiving, processing and transmitting control unit, at least one actuator which is responsive to signals from the control unit and can adjust the clutch to select the rate of torque-transmission, and means (such as one or more sensors, electronic and/or other circuits, and one or more conductors) for transmitting signals to the input or inputs of the control unit. Furthermore, the power train comprises means for starting or operating the prime mover (e.g., an electronic circuit which is connected with the control unit by a bus and/or other suitable signal conducting means). It is also known to cause the control unit to shift from a standard or normal mode of operation to an emergency or standby mode when one or more constituents of the signal transmitting means become inoperative or defective, either entirely or in part. For example, the condition of the control unit can be changed from the standard mode to the emergency mode in response to a breakdown of a sensor, a circuit and/or a conductor forming part of the signal transmitting means.
A control unit having standard and emergency or standby modes can be utilized in power trains which embody automated clutches and/or automated transmissions and serve to transmit motion from the prime mover (e.g., an internal combustion engine or a motor) to the driven wheels of the motor vehicle. In accordance with a presently known proposal, the control unit is set up to automatically engage the clutch if the prime mover of the motor vehicle is arrested while the transmission is in (a forward or reverse) gear. This causes the clutch to act as a parking brake when the engine is turned off while the transmission is in a gear other than neutral. Thus, the drag torque of the engine prevents the vehicle from carying out an undesired rolling movement when the engine is off, irrespective of the condition of the vehicle brake or brakes.
However, the just described conventional power trains also exhibit certain serious drawbacks. For example, if the motor vehicle is brought to a halt on a sloping surface, i.e, if the clutch is automatically engaged when the engine is off but the transmission is in a forward or rearward gear while the wheels contact an upwardly or downwardly sloping road surface, the power train is stressed so that a shifting of the transmission from a forward or reverse gear necessitates the exertion of a substantial effort or cannot be carried out at all.
Reference may be had, for example, to published German patent applications Nos. 195 04 847 and 195 48 799 which disclose that the control unit receives signals from sensors which monitor the condition of the transmission (i.e., the selected gear ratio) and/or serve to transmit signals denoting the intention by the operator of the motor vehicle to shift from a previously selected gear into a different gear. An advantage of the signals denoting the intent of an operator or driver to shift the transmission into a particular gear (e.g., as a result of the application of a certain force to a gear shifting lever or the like) is that the clutch can be intentionally disengaged in good time, for example, prior to actual shifting of the transmission into a different gear. Thus, in normal operation, the engine can be activated or actually started while the transmission is in neutral gear. However, if, in normal operation of the power train, the engine can be started only while the transmission is in neutral gear, the transmission cannot be shifted into neutral gear (from the reverse gear or from a forward gear) when the sensor or sensors which monitor the condition of the transmission and/or the operator's intent to shift the transmission into a selected gear is or are defective and/or the connection(s) between such sensor or sensors and the control unit for the clutch is or are defective. The reason is that, in the event of such defectiveness, the clutch is not automatically disengaged and, therefore, the engine of the motor vehicle cannot be activated or started for the purpose of setting the motor vehicle in motion, for example, for the purpose of causing the vehicle to perform a crawling movement. In other words, the motor vehicle is no longer capable of moving, and this solely as a result or consequence of a defectiveness or malfunctioning of one or more sensors and/or circuits and/or connection or connections between such defective or malfunctioning sensor or sensors and/or circuit or circuits on the one hand, and the circuitry of the control unit for the automated clutch on the other hand.